xrandr: can't open display

I am having trouble with my [7-inch type C LCD screen](https://www.waveshare.com/wiki/7inch_HDMI_LCD_(C)).  ; When I try to display an application on it, I get the error Missing X server or $DISPLAY.  I use the command xrandr and get the message can't open display.  I found some solutions for Raspberry, but my Orange Pi lite board doesn't have config.txt. I tried adding the line disp_mode=1024x600p60 to armbianEnv.txt, but that doesn't seem to be the solution.  I currently use Armbian Minimal (a stripped-down version).  Could that be the problem?  xrandr -display :0.0 returns Can't open display :0.0.